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December 2017 • SMT Magazine 73 EVALUATION OF STENCIL TECHNOLOGY FOR MINIATURIZATION The coupon has 11 rows with various aper- ture openings in terms of size and shape, repre- senting various components. Each row has 10 identical apertures to provide an indication of repeatability. The first row is a circular aperture with a specified diameter of 7.3 mils and the 2nd row is a square aperture with a specified length and width of 7.3 mils. These two rows are nearest the typical aperture for a 01005 component. The top side and bottom sides of the coupon were measured at Vicor using a Keyence digital microscope. The location of the aperture relative to the fiducial, or location accuracy, was not measured in this study. Each measurement was ranked as target condition (green), acceptable (yellow), or unacceptable (red). A green aperture measured within ±.3 mils to the specification, a yellow was within ±.3 to .5 mils, and red was outside of ±.5 mils. The number of green, yellow and red ratings for each stencil was tabulated and the stencils ranked based on this criteria. The most accu - rate stencils did not have red ratings, and were ranked based on the highest number of green and fewest number of yellow ratings. The sten- cils with red ratings were ranked in descend- ing order based on the number of red ratings. Table 2: Stencils ranking by aperture size accuracy. Figure 2: Stencil coupon for F18. If two stencils had the same amount of red rat- ings, the number of green and yellow ratings was used to delineate the ranking. The summa- ry appears in Table 2. Visual Assessment of Print and Stencil The visual assessments of paste deposits af- ter the first and sixth print were performed at two selected fine pitch component pads 0.3 mm microBGA and 0.4 mm microBGA locations. Each stencil was also inspected after the final (sixth) print to have perspective on the amount of paste remaining in the apertures. The pho- tos of uncleaned stencil apertures/windows at the same two component locations were also re- corded after last or 6th print. Based solely on vi- sual inspection, the stencils were grouped into the three categories shown in Table 3. The photos of typical square shape aper- tures both regular and with radiuses corners are shown in Figure 3.